1. Field of the Invention
The present invention relates to a capacitive sensor for detecting a physical value such as acceleration, and more particularly to a capacitive sensor in which a pair of plates having electrodes are bonded to each other by an anodic bonding method.
2. Discussion of the Prior Art
A capacitive acceleration sensor is composed of a pair of plates each having an electrode. The first plate has a fixed portion and a movable portion on the latter of which a moving electrode is formed. The second plate has a fixed electrode. The two plates are anodically bonded to each other so that two electrodes faces with a predetermined gap.
In such sensor, a plurality of spring beams are formed between the fixed portion and the movable portion of the first plate so that the movable portion is supported by the fixed portion through the spring beams. Namely, one end of each spring beam is connected to the movable portion and the other end of each spring beam is connected to the fixed portion.
When an acceleration is applied to the capacitive acceleration sensor, the spring beams deflects toward the second plate to change the gap between the moving electrode and the fixed electrode. Therefore, the capacitive acceleration sensor can detect the acceleration by measuring the change in the gap between two electrodes i.e. the change in capacitance C between two electrodes. The capacitance C is generally expressed as follows: EQU C=.epsilon.S/d
where, .epsilon. is a dielectric constant, S is the area of the electrodes and d is the gap between the electrodes.
When both of the plates are anodically bonded to each other, an electrostatic force Pe is generated therebetween. This electrostatic force Pe is derived as follows: EQU Pe=.epsilon.SV.sup.2 /2d.sup.2
where, .epsilon. is a dielectric constant, S is the area of the electrodes, V is a voltage applied to the electrodes and d is the gap between the electrodes.
The above equation means that the electrostatic force Pe increases in proportion to the squared value of the voltage V which is applied between the plates for anodically bonding them. When a electrostatic force Pe acts between the plates, the spring beams deflects toward the second plate and finally sticks thereto because the spring beams are very flexible.
The above-described capacitive acceleration sensor has a three-stratum structure. Namely a first plate made of silicon is placed on a substrate made of glass, and a second plate made of glass is then placed on the first plate. These substrate and first and second plates are anodically bonded as follows.
At first, a voltage of 800 V is applied between the first plate and the second plate to anodically bond them. During this fabrication process, the charge stored on the surfaces of both plates generates a great electrostatic force Pe therebetween. The spring beams formed on the first plate then sticks to the surface of the second plate facing the first plate. In this state, the gap between the electrodes does not change even when an acceleration is applied to the capacitive acceleration sensor thereby making it impossible to detect acceleration.
Further, a voltage of 900 V applied between the first plate and the substrate to anodically bond them. During this fabrication process, the charge stored on the surfaces of the first plate and the substrate also generates a great electrostatic force Pe therebetween. A weighted portion suspended with the moving electrode then sticks to the surface of the substrate facing the first plate. In this state, similarly, it becomes impossible to detect acceleration.